The highest fine for fraud ever imposed by the SFO is £2.2m, she said, while in the US the figure is $3bn. The UK should strengthen its penalty regime so that it is "more in line with other jurisdictions", Thornberry added.

The common perception, Thornberry said on Friday, was that "when a man steals your wallet, he gets a stretch in prison; when he steals your pension, he gets away with it".

One barrier to prosecuting companies for fraud is the requirement that individual directors must be identified as being directly involved in ordering or committing an offence.

"We must change this," she told the Society of Labour Lawyers. "We should hold companies vicariously liable for their employees' crimes." The principle already exists in the Bribery Act, which Labour pioneered.

"US attorneys I have spoken to all agree that vicarious liability is the key to success. We can't continue to allow companies to say: 'We had no idea this was happening' when it's a question of the culture" of the firm.

But the coalition government was now trying to water down the provisions of the act, claiming that it imposes too many restrictions on small businesses, the shadow attorney general said.

The maximum penalty for money-laundering is 14 years but for fraud only 10 years. Labour is examining increasing the sentence for fraud to 14 years.

Even the European commission levies fines based on a percentage of a firm's turnover. "We should be thinking about doing that," she commented.

The SFO, which has had its funding cut by 25% under the coalition government, should be allowed to keep a proportion of the fines it collects to provide greater resources and give investigators an incentive to hunt down criminals.

Labour's policy document says: "In the US, assets confiscated by prosecutors as proceeds of crime are put into the department of justice's asset forfeiture fund. Wherever possible, confiscated money is returned to victims.

"However, this system does result in hundreds of millions of dollars being spent each year on financial investigators, forensic accountants and computers. As result, the leading fraud divisions in the US criminal justice system bring in many times the salaries of the individuals who work for them."

Thornberry said: "Because of the shortage of money, the SFO is having to go to the Treasury in secret for any investigation which cost more than £1.5m.

"This gives George Osborne a secret veto on fraud investigations. It does not look to me as though justice is being done if it's down to the Treasury" to decide.

Recent parliamentary answers by ministers have insisted that it is the SFO's director who decides whether to launch an investigation.

Labour has already promised to bring in an economic crime bill if it comes to power at the next election. Its document declares: "Nothing does more to fuel cynicism and resentment about politics than the perception that there are elites who are above the law and who rig the system for themselves."